Electrical control apparatus



Feb. 1, 1944. A. WINTHER ELECTRICAL CONTROL APPARATUS Original Filed May 5. 1941 5 Sheets-Sheet 1 'L-I m FIG. I.

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Feb. 1, 1944. A. WINTHER Re. 22,432

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Reislued Feb. 1, 1944 Anthony Wlnther. Kenoshs, Win, asslg'nor to Martin P. Whither, as trustee Original No. 2,286,778. dated June 18, 1942. Se rial No. 891,933, May 5, 1941. Application for reissue August 19, 1942, Serial No.' l55,851

19 Claims. (CL 172-284] This invention relates to electrical control apparatus, and with regard to certainmore specific features to torque control apparatus for alternating-current motors.

Among -the several objects of the invention may be noted the provision of means for setting a limit upon the torque to be delivered from a motor, this torque to be at a determined ratio to full motor torque; the provision of apparatus of this class which, after torque is set, requires no further manual adjustment in order to obtain conditions of minimum time for accelerating a driven machine up to running speed; and the provision of apparatus of the class described which permits of accelerating the driven machine under conditions of complete safety to the driving motor. Other objects will be in part obvious and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction, and arrangements of'parts which will be exemplified in the structures hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings, in which are illustrated several of various possible embodiments of the invention,

Fig. 1 is a diagram of mechanical apparatus forming part of the invention;

Fig. 2 is a wiring diagram of a torque-control circuit for Fig. 1;

Fig. 3 is a fragmentary view similar to Fig. 1,

Fig. 4 is a wiring diagram similar Fig. 2, but showing an alternative circuit arrangement applicable to Fig. 3; and,

Fig. 5 is a wiring diagram of an alternative circult for carrying out the invention.

adjustment by the operator. This will permit the centrifuge machine to accelerate in the least possible time consistent with safe loading of the motor.

In cases of A. C. motors coupled to their loads through means which maintain a predetermined constant output speed, it is frequently desirable to limit the torque which the motor can apply to machines operating under varying loads, and also to function as a safeguard against extreme overloading when starting.

, The invention is also useful tolimit or control torque in cases where plastics, glues and the like are stirred where. upon thickening, the load due to stirring might increase to undesirable values so far as the motor is concerned, but where slowing down of the stirring could be tolerated along with limitation of motor torque if there were ab- 7 soluteassurance .against complete cessation of movement. The present invention gives this assurancaby assuring that the motor cannot be loaded to a stall condition.

Referring now more particularly to Fig. 1, there is shown at numeral l a combination electric motor and an electromagnetic eddy-current slip clutch unit consisting of an A. C. motor element 3, the rotor 4 of which is keyed on a rotary quill 5. This quill 5 is mounted upon bearings I which in turn are carried upon a shaft 9. Shaft 9 forms the driven element and is rotary in bearings I lin a case ll. A variable magnetic slip coupling is maintained between the quill 5 and the shaft 8 by means of an electromagnetic eddy-current slip clutch it, having an eddy-current drum 6 and an A. C. fleld coil indicated at CL. The coil CL is carried upon a toothed rotor 8 keyed to shaft 9. This coil is connected across slip rings l'l over which current is brought to energize the coil to effect a slipping magnetic coupling.

Motor-driven clutch apparatus of the above form is particularized more fully in the United I States Patent 2,286,777 of Martin P. Winther torque of a motor the maximum torque which is applied by that motor, particularly'in the case of A. C. motors. This is useful in various appli-,

'et al., dated June 16, 1942. for-speed control unit.

Referring now more particularly to Fig. 2, there is shown at L-I L-2, L-I a three-phase, A. C. circuit which supplies the A. C. motor 8. At CL is the slip clutch winding which provides a D. C. clutch load, upon clutchmasnetizatlon. The

direct current for this load is supplied through a pair of three-element, hot-cathode, gas-filled rectifler tubes RT of the half-wave type.

At AT is an anode transformer which supplies powerfor the D. C. clutch load. this transformer AT being connected on its primary side across line operates primarily to cannot short circuit the former GT.

motor circuit is always 180 wires L-l and L-2 of the A. 0. supply circuit. Anodes A of the tubes RT are shown connected to the opposite ends of the secondary of the anode transformer AT.

Tube cathodes or-heaters K are energized by a filament transformerKT, the primary of which transformer is also energized from line wires L-i and L-2. The filament transformer KT is regulated by means of a filament rheostat FR.

The grids of the tubes RT are shown at G. Resistors 1BR are grid current-limiting resistors, whereby the grid currents are held to a low value of two or three micro amperes at the maximum. A suitable value for these resistors BR. is 50,000 ohms.

The opposite ends of the secondary of the transformer KT are respectively connected across the cathodes K of the rectifier tubes RT.

The secondaries of the anode transformer AT of the cathode transformer KT are connected together at their center taps. as shown, through the clutch coil CL which provides the D. 0. load. Thus the D. C. load through the clutch coil CL may flow, when permitted, between the center taps of transformer AT and transformer KT and through the rectifier tubes RT alternately as the respective tubes RT are alternately fired or released by the grids G to pass current in a manner to appear.

At GT is shown a grid transformer which supplies practically no current to the system, but place alternating potentials upon the grids G to cause alternate firing or current release through the tubes RT. This transformer GT has its primary also connected across the supply lines L'! and L-2, as shown. The v ltage of the secondary of the grid transformer GT is approximately 100 volts, but this may be adjusted in accordance with requirements of the circuit. Resistors GB, for example, of 10,000 ohms each, and themselves in series, are connected across the secondary of the transformer GT. They hold the current in the secondary to a low value. The result is that the potentials in pposite ends of the two resistors GB are applied to the grids G through the resistances BR. A

center tap between the resistances GB is connected to the center tap of former KT so as to form a the filament transbasis for a. grid-tcathode potential relationship.

In the motor lead L-3 are placed one or two turns of wire W, used as a able potentiometer P which can he graduated or set by anpperator. Thi setting may be made in v view ofthe reading of an A resistance R1. of 10,000. ohms is inserted in the potentiometer circuitand across the secondary of transformer GT sothat transformer LT secondary of the translimi It will be noted that the grid transformer GT sets up a potential which is posite wave from legs L-l and L-I. inherently the electrical wave in leg L-3 Since of the out of phase with the based upon acomgrid G becomes negative.

The potential from transformer LT is applied at some value, according to the setting of the former LT will exactly neutralize first one'and then the other of the potentials on resistors GB. Since the potential supplied by transformer LT is proportional to the current flowing in the legs of the motor circuit (particularly leg L3), the

seen that, as long as the current of the'A. C. motor is lower in value than that, which is necfore, the ammeter AM, curacy, indicates the value of torque (according to some constant) at which cut off of energization to CL will occur.

It will be clear from the above that the rectifying tubes RT will not fire, and thus not feed it will be seen that the "of the tubes may be cut oil at any desired current value in the line L-3.

Also, the adjustment of the potentiometer r. .eostat P need not be left invention consists in controlclutch l5. the shaft 8. Fig. 3 shows such a governor on v are current from rectiflers. The rectiflers are op higher anode voltage than the two inner tubes erated to fire, and energize the clutch field, by

rectifying from one portion of the motor supply circuit. An out-of-phase regulating voltage from another portion of the motor circuit is impressed upon the grids of these rectiners a1- temately and definitely to stop their-firing, and thus to stop clutch ileld energization, when the currentvalue in the motor A. 0. supply circuit rises to a desired point. The load referred to in the appended claims is the workload on shaft and which is supplied by motor torque. The

load for energizing coil CL is not supplied by the motor but directly from circuit I'r-r', L-,-2,

Tests made show, for example, that if the normal current at full load torque of the motor is 100 amperes and the motor is started while thepotentiometer P is adjusted until the ammeter shows 100 amperes, the motor will continue to draw not over 100 amperes. Furthermore, it will draw very little under that value continuously .until the motor is fully accelerated. This means in the case of a centrifuge, for example, that the motor will continuously apply torque to the maximum predetermined allowable value, which means that the load will be accelerated in the least possible time consistent with the desired motor loading. Time of acceleration may be controlled by adjusting P.

As indicated in said patent above mentioned, a speed-responsive governor may be used in connection with the driven element of the For example it may be driven by shaft 9. Its function is to control contacts or a carbon pile or the like to open upon increase in speed and vice versa. If it is desired to use a speed control in connection with the present apparatus, the contact circuit of the governor is connected in one of the lead lines to the transformer GT, as shown in Fig. 4. Irrthis Fig. 4, the governor is shown I at GV, consisting of a resistance in parallel with a suitable condenser. This resistance is also in parallel with-contacts diagrammatically shown at N which respond -by opening to overspeeding of the shaft 9. Thus the governor' simply reduces current in, and causes the potential available out of the transformer GT, to be cut ofl when the clutch shaft 9 tends to overspeed, and to be re-established when the clutch shaft 9 tends to run under the desired speed. The remainder of the circuit which functions for torque limitation is still used in the way above described in connection with Figs. 1 and 2 to set a point beyond which the torque of the motor cannot rise. The duty of the governor is very low in watts, inasmuch as the only wattage required is the very small amount required to flow in the resistors GB.

It may be seen that, although the rectifying tubes RT are each half-wave tubes, if taken together they constitute. a full-wave rectifying means.

It. will be noted that the rectifier tubes RT are connected in parallel to feed current into the clutch coil. A voltmeter VM may be used to deponents, and also employing four tubes RT-l,

RT2, RT-3 and RT-l' firing into the coil CL. The two outer tubes RT-l and RT-l have a and ordinarily take the load, as will appear.

In Fig. 5,'Ll, L-2, L-I again is the threephase supply circuit for the motor 3, and lines L--| and L- -2 supply the transformer AT. The four tubes RIF-i, RT-I, RT--3 and RT-l are connected to the secondary of the transformer AT, as shown, equidistantiy by pairs on opposite sides of the center tap connection for the coil CL. The other side of the coil CL is connected to the mid taps of the transformers KT which are also energized from the lines L-l and L! as before. The cathodes or heaters are again indicated at K. The grids are indicated at G. The resistances BR are indicated in connection with the-respective grids G. The anodes ofthe tubes are identified as A. Thus it will be seen that the primary change in the principal tube circuit is the provision of four tubes, two of which ordinarily take the load. The more fundamental change is in the provision of direct current in the control circuit, instead of alternating current as in Figs. 1-4. Three basic control circuits are used, and are indicated in dotted boxes identified .generally by numerals I, II and III.

The reason for using D. C. control circuits is to eliminate, in certain instances, design difllculties;

in regard to phasing, arising from the possible efiect of power factor, or incorrect connections which might easily be made. For example, it is possible to obtain transients when a governor is used, which might confuse an erector in properly phasing the circuit. Thus the following description refers to the same general torque control circuit and governor circuit as already illustrated in Fig. 4, broadly speaking, but use is made of direct current for the various control functions, instead of alternating currents.

The control circuit 1. may be referred to as the regardless of any transients that-might otherwise cause the two inner tubes to function when not wanted. Therefore, the normal continual function of the two outer tubes RT-l and RT-.4 is assured not only by the higher anode voltage but byia less quantity of negative bias on the grids.

Tracing through circuit I, the direct current from the rectifier Rl is passed through a filter circuit containing condensers I2l and l3l with a choke H I. interposed between connections of the condensers in the conventional manner. Potentiometer resistor PT causes a steady load of a very small magnitude on rectifier R-l.

From the resistance PT, at point 29, a negative voltage of the value minus twelve -12), for example, is taken off and imposed on the grids of the outer tubes RT--i and RT-l. At the point 28 a larger negative voltage value, of minus sixteen (16), is taken off and imposed on the grids of the two inner tubes R.T-2 and RT-l. Normally, the negative bias described and established by negative-bias circuit I will prevent the tubes from firing so that-no direct current will be supplied to the clutch CL.

The rectifier R-Z in the governor-bias circuit II is arranged so that direct current is produced only when the contacts N in govemor'G V close, or in any event a very small amount may be supplied by resistor GR, which is insufficient appreciably to affect the circuit because GR' is of a high value. Resistor LR is preferably placed across the primary of transformer 20 so as to control load on the governor contact points to permit them to pass current and keep themselves clean. The positive output of governor rectifier R-2 is connected to the positive output of the circuit I at point ll. The negative side is connected (via resistance TBB) to the top of abias blocking resistor BB in such a manner as to place a negative charge on a point 8| of this resistor BB when the governor contacts close.

Assume that the governor contacts are open and no current flows in rectifier R-2. Rectifier R-l supplies a steady D. C. potential. .Therefore, all metallic conductors and elements connectedto point 5| in circuit I will have a positive value, and because there is no appreciable cur-,- rent flow, due to the high value of the blocking resistors BR in the grids of the tubes, a substantially uniform positive condition will occur in all such metallic conductors or elements. Hence, the cathodes K in the tubes will have a certain positive value while the grids in each tube, being connected to points 28 and 29 on potentiometer PT will all have a normal and steady negative value, thus preventing the tubes from firing.

, Now assume that the governor contacts close in circuit II. A negative potential, or a drawingaway of current, will occur through resistor TBB, point 9|, and point 81. Hence the closing of governor points will cause a subtractionof positive at point 8|. Hence, the difference of potential between cathodes K and the grids will inter? mittently be reduced so that the tubes can fire. Theresistor BB is necessarily made rather high in value, as shown, which is 10,000 ohms, so that this potential cannot instantaneously rectify itself, or balance out. Therefore, it can be seen that the closing of the governor points causes the tubes to fire. The characteristic of these tubes is, as before, to fire as long as the anode potential remains in the positive area of the cycle or sine wave under consideration. 4

- Regarding torque bias circuit 111, it is supplied from transformer 23. The primary of another transformer 2| provides inductive load for the current transformer 23. Transformer 2| supplies the rectifier R3. A resistor 22 establishes a uniform load on the output of the current transformer 2| so that at no time, regardless of the adjustment of potentiometer P-l, can there be a condition when the output is not loaded.

Potentiometer P-| may be adjusted to place a certain needed potential for the damping-out or stopping of the firing of the tubes in response to the value ofthe current in amperes in line 11-3. It will be noted that the positive terminal of rectifier R-3 connects to point 8| via point I]. In this case the source of the positive in rectifier R3 has the positive on point 8|. Resistor TBB in this case provides a load for rectifier R-3. It also establishes and helps maintain a relatively high positive at point 9!, because of its value. also be noted that resistor T33 is half the value of resistor BB. The reason is that resistor 'I'BB must permit enough negative effect through points llILll, and to 8! via ll, so as to permit the governor to function, but the resistor TBB also functions to permit the passage of positive from rectifier R-3 to point 8| via point 8|, despite the load across TBB on rectifier R-l. Therefore, when the current in supply line L-I rises to such a. value as will cause a positive voltage from rectifier R-dto re-establish the carelessly left there possibly turn on all 5. 3 cuit III is a means for guarding against such a ic actuated by coil 33.

function of re-establishing It should I may be inserted in the no fire potential valuecn point 8 I, then regardless of the opening and the closing of the governor the tubes will not fire. Thus the governor speed control is overriden by the torque-bias control. In this manner, the torque control circuit III can prevent operation of the tubes when the desired current into the motor is exceeded by overloading.

Not only does the torque control circuit III provide means for limiting the torque the motor can exert, but provides a safety feature, so that the motor {cannot be stalled, due perhaps to some condition where the governor, for example, has been set at some high speed condition and by the operator who may circuits at once without assuring himself that the governor has been turned to zero position.

In the case of large motors coupled with eddycurrent clutches, the inertias of the motor rotor and the clutch elements may be high. Thus applying full excitation, as would occur if the governor were at rest and set for some high output speed, would cause the motor 3 to stall because it could not develop enough torque to accelerate the masses as quickly as the eddy-current clutch coupling would require. The torque control cirstalling condition.

As a further protection, a time delay relay 2 Y line which supplies AT, so that"contacts SW thereof are caused to remain open until closure time delay element T-I ,This relay delays the time at which the anode voltages are applied, even though all the cathode transformers have been previously energized directly from lines L-|, L-2. Thus the transformers KT are energized, and one minute later closes.

Also, another contact SW| may be inserted in one leg of current transformer 23 to protect against the high inrush into motor 3 when the line switch LSW is closed to the motorcommonly known that induction motors take as high as seven times the normal full load current when'they are connected across ,the line. Such a condition might possibly burn out rectifier R 3 because of heavy over-normal voltages applied to it. Therefore, contact SWI is placed in the circuit of transformer 23 and closes one tacts andoperating parts SW- and SW| are being known deshown only diagrammatically, vices, per se.

Additional advantages of the D. C. circuits described in Fig. 5 are that no phasing of transthe time delay switch SW former III is necessary, nor is phasing of any other portion of the circuit necessary. It is also found that the vacuum tubes are more stable when direct current is used. Rectifiying vacuum tubes may be used in place of rectifiers R-l, 3-2 or R-l.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. As many changes could'be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. A

I claim:

1. Torque control apparatus for a motor energized from an A. C. circuit comprising an eddycurrent clutch between the motor and a load, a D. C. field coil in said clutch, A. C. rectifier means feeding said D. 0. field coil, potential means reof rectified current from the respective tubes to said field coil, and means responsive to current sponeive to one portion of said A. C. circuit for" actuating the rectifying means to rectify, and a second potential means responsive to current in said A. C. circuit and connected in opposition to said first-named potential means for neutralization against rectification at predetermined current values in said A. C. circuit, whereby the clutch coil is energized only up to current of predetermined value in said A. C. circuit.

2. Torque control apparatus for a motor energized from an A. C. circuit, comprising an eddycurrent clutch between the motor and the load, a D. C. field coil in said clutch, rectifying means connected to said coil and energized from said A. C. circuit, potential control means for said rectifying means operative from one part of said A. C. circuit, a second potential means operative in response to current fiow in another part of said A. C. circuit for neutralizing at predetermined current values the first-named potential,

whereby at said currents of predetermined values rectification and the delivery of current to said coil is stopped, and variable potentiometer control means adapted to effect neutralization of relatively more potential in response to less current, and vice-versa.

3. Torque control apparatus for an A. C. motor energized from an A. C. circuit, comprising an eddy-current clutch between the motor and a load, a D. C. field coil in said clutch, rectifying means adapted to feed direct current to said clutch winding, means for supplying energy to said rectifying means and the clutch coil from one part of the A. C. circuit which serves said motor, means energized from said section of the motor circuit above mentioned for placing a grid potential upon the rectifying means to control current fiow therethrough, and means responsive to another part of the A. C. motor circuit and in opposite phase with respect to said grid potential, whereby current flow through the rectifying means is blocked when the current in the A. C. circuit exceeds a predetermined value.

4. Torque control apparatus for an A. C. motor fed from a three phase circuit comprising an eddy-current clutch, a field coil for energizing the clutch, a pair of three-element vacuum tube rectifiers connected to feed direct current in parallel into the field coil, means energized from the three phase circuit to feed alternating current to said tubes, means responsive to alternating current from one portion of the motor circult adapted to apply biasing potentials alternately to the grids of the tubes toalternate flow in another part of said A. C. motor circuit adapted to apply in opposite phase a potential against 1 said biasing potential to prevent flow from both tubes in response to excessive current in the A. C. circuit.

5. Torque control means for an A. C. motor in a three phase 'A. C. circuit comprising an eddy-current clutch between the motor and the load, a field coil in the clutch, a pair of vacuum rectifier tubes connected to feed in parallel to the field coil, each of said tubes having an anode, a cathode, and a grid, anode and cathode transformers respectively connected to the anodes and cathodes and energized from said motor circuit, a grid-bias transformer connected to apply alternately to said grid potentials to fire the tubes in circuit with the clutch coil, said grid-bias transformer means being connected to one portion of said three phase circuit, and a fourth transformer responsive to current in another portion of said three phase circuit which is opposite in phase to the supply for the grid-bias transformer and connected in potential opposition to said grid-bias transformer.

6. Torque control means for an A. C. motor in a three phase A. C. circuit comprising an eddycurrent clutch between the motor and the load, a field coil in the clutch, a pair of vacuum rectifier tubes connected to feed in parallel to the field coil, each of said tubes having an anode, a cathode, and a grid; anode and cathode transformers respectively connected to the anodes and cathodes and energized from said motor circuit, a grid-bias transformer connected to apply alternately' to said grid potentials to fire the tubes in circuit with the clutch coil, said grid-bias transresponsive to current in another portion of said three phase circuit which is opposite in phase to the supply for the grid-bias transformer and connected in potential opposition to said gridbias transformer, and potentiometer means for adjusting the potential delivered by said lastnamed transformer, whereby more or less motor current can be made to correspond to firing of the tubes, as desired.

7. Torque control apparatus for a motor energized from an A. C. circuit, comprising an eddy-current clutch between the motor and a load, a D. C. field coil in said clutch, a pair of half-wave rectifying tubes feeding in parallel into said coil, each tube having an anode, a cathode and a grid, an anode transformer having a secondary connected to the anodes, a cathode transformer having a secondary connected to the cathodes, said transformers having primaries connected to said A. C. circuit, a grid transformer having a primary connected to one part of said A. C. circuit and a secondary connected to said grids and to said cathode transformer, to effect alternating potential on. said tubes for altemately causing current to fiow into the clutch coil, a current transformer having a primary connected to another part of said A. C. circuit and having a secondary connected with sa d grid transformer in potential opposition. potentiometer mean across the last-named secondary for controlling the potential opposition. whereby predetermined current in the motor will cause redetermined neutralization of potential on said grids, so that motor current and torque are automatically limited by the resulting slip- 'glzedfromanLC.

ping in said clutch when current from said tubes stops, and speed responsive means in the primary oi said grid transformer responsive to the speed of the loadslde of said clutch to demergize the grid transformer and consequently at excessive speeds causing said tubes to stop their current output.

8. Torque control apparatus for a motor enercircuit, comprising an eddycurrent clutch between the motor and its load, a D. C. field coil in said clutch, means feeding said D. C. field coil and having a control responsive to potential, D. C. potential means responsive to one portion of said A. C. circuit and including a rectifier for controlling said feeding means, and a second D. C. governor-controlled potential means including a second rectifier responsive tosaid A. C. circuit and connected to affect said first-named potential and adapted to superimpose governor control on the feeding means.

- 9. Torque control apparatus for a motor energized from an A. C. circuit, comprising an eddy: current clutch between the motor and its load, a D. C. field. coil in said clutch, means feeding said D. C. field coil and having a control responsive to potential, D. 0. potential means responsive 'to one portion of said A. C. circuit and including a rectifier for controlling said feeding means, a second D. C. governor-controlled potential means including a second rectifier responsive to said A. C. circuit and connected to affect said first-named potential and adapted to superimpose governor control on the feeding means, and a third potential means responsive to excessive values of current in said A. C. circuit to override governor control and to affect said bias circuit connected first-named potential.

10. Torque control apparatus for a motor energized from an A. C. circuit comprising an eddycurrent clutch between the motor and a load, a D. C. field coil in said clutch, rectifier tubes connected to said coil and energized from said A. C.

circuit, means for maintaining a negative bias on said tubes comprising a D. C. negative-bias circuit connected with the grids of said tubes, a rectifier, and means for feeding current from said A. C. circuit to said D. C. biasing circuit through said last-named rectifier, said bias tending to prevent the tubes from energizing said field coil, a governor driven by the clutch, a governor circuit energized from said A. C. circuit, a rectifier in said governor circuit, a D. C. portion in said governor circuit and fed from its rectifier, said governor upon overspeeding opening its contacts to deenerglze said governor circuit whereby the biasing means is unafifected and does not cause said but when said governor contacts close upon underspeeding the governor circuit is energized to affect said biasing means to cause the tubes to energize said coil.

11. Torque control apparatus for a motor en ergized from an A. C.,circuit comprising an eddycurrent clutch between the motor and a load, a D. C. field coil in said clutch, rectifier tubes connected to said coil and energized from said A. C. circuit, means tor maintaining a negative bias on said tubes comprising a D. C. negativewith the grids of said tubes. a rectifier, and means forieeding current from said A. C. circuit to said D. C. biasing circuit through said last-named rectifier, said bias tending to prevent the tubes from energizing said field coil. a overnor driven by the clutch,

tubes to energize said field coil,

a governor circuit energized from said A. C. circuit, a rectifier in said governor circuit, a D. C. portion in said governor circuit and fed from its rectifier, said governor upon overspeeding opening its contacts to deenergize said governor circuit whereby the biasing means is unafiected to cause said tubes to energize said field coil, but when said governor contacts close upon underspeeding the governor circuit is energized to afi'ect said biasing means to cause the tubes to energize said coil, and a D. C. torque-bias circuit energized from the A. C. circuit through a rectifier and connected to override the control by the said bias circuits to cause the rectifier tubes to feed the field coil only when predeterminately low curents fiow in said A. C. circuit. 12. Torque control apparatus for a motor energized from an A. C. circuit comprising an eddycurrent clutch between the motor and a load, a D. C. field coil in said clutch, at least four rectifier tubes feeding said field coil, a biasing circuit connected to the grids of said tubes for bias tube control, said biasing circuit being connected to'said A. C. circuit and comprising a rectifier, a potentiometer connected with said rectifier, a connection with the potentiometer arrangedat one series of potentials and controlling two of said tubes and a second connection connected with said potentiometer arranged at another series of potentials and controlling the other two tubes, a governing biasing circuit comprising a governor electrically connected with said A. C. circuit, and a rectifier fed thereby, said governor 'biasing circuit having a D. C. portion connected '5 with said first-named biasing circuit to apply an additional bias tube control.

13. Torque control apparatus for a motor energized from an A. C. circuit comprising an eddycurrent clutch between the motor and a load, a D. C. field coil in said clutch, at least four rectifier tubes feeding said tubes, a biasing circuit connected to the grids of said tubes for bias control, said biasing circuit being connected to said prising a rectifier, a potentiometer connected with said rectifier, a connection at one potential with the potentiometer and controlling two of said tubes and a second connection connected with said potentiometer and controlling the other two tubes at a difi'erent degree, a governing biasing circuit comprising a governor electrically connected with said A. C. circuit, a rectifier fed thereby, said governor biasing circuit having a D. C. portion connected .with said first-named biasing circuit, and a torque-biasing circuit comprising an A. C. section energizing from said A. C. circuit, a rectifier in said torque-biasing circuit, and a D. C. portion in sald torque-biasingcircuit fed by its rectifier and connected with both of said previously mentioned torque-biasing circuit being responsive to rise in current in the A. C. circuit to maintain a condition of potential on the grids to prevent tube firing regardless of the action of the governor cirbiasing circuits. the

14. Torque control apparatus for a motor energized from an A. C. circuit, comprising an eddy-current clutch between the motor and the load, a D. C. field coil in said clutch; rectifier tubes energized from the A. C. circuit energizing said field coil with direct current, control grids in said tubes, a potentiometer connected with said grids: a negative-bias circuit feeding said potentiometer, said negative bias circuit being energized from the A. C. circuit and including a field coil, grids in the v A. C. circuit and com-.

rectifier. negative bias from said negative bias 'circuit normally being such as to prevent the tubes from energizing the coil; a governor-bias circuit having a contact governor therein responsive to the driven member of the clutch to open and close its contacts, said governor-bias circuit also being energized from the A. C. circuit and including a rectifier, said governor-bias circuit being so connected to the rectifier in the negative-bias circuit that when the governor points are open the bias on said grids remains such as to prevent the tubes from energizing the coil, but when the governor contacts are closed, the coil will be energized; a torque-bias circuit also energized from said A. C. circuit and including a rectifier, said torque-bias circuit being connected with the said two bias circuits so that upon undue rise of current in the A. C. circuit supplying the motor, said rectifier tubes will be prevented from energizing the coil regardless of the open or closed condition of said governor p ints.

15. Torque control apparatus for a motor energized from an A. C. circuit, comprising an eddy-current clutch between the motor and the load, a D. C. field coil in said clutch, rectifier tubes energized from the A. C. circuit energizing said field coil with direct current, control grids in said tubes, a potentiometer connected with said grids; a negative bias circuit connecting with said potentiometer, said negative bias circuit being energized from the A. C. circuit and including a rectifier, said potentiometer providing less negative bias for some oi said tubes than others, the negative bias from said negative bias circuit normally being such as to prevent any of the tubes from energizing the coil.

16. Torque control apparatus for a motor energized from an A. C. circuit, comprising an eddy-current clutch between the motor and the load, a D. C. field coil in said clutch, rectifier tubes energized from the A. C. circuit energizing said field coil with direct current, control grids in said tubes, a potentiiometer connected with said grids: a negative bias circuit connectin with said potentiometer, said negative bias circuit being energized from the A.'C. circuit and including a rectifier, said potentiometer providing less negative bias for some of said tubes than others, the negative bias from said negative bias circuit normally being such as to prevent any of the tubes from energizing the coil; a governorbias circuit having a contact governor therein responsive to the driven member of the clutch to open and close its contacts, said governorbias circuit also being ener ized from the A. C. circuit and including a rectifier, said governorbias circuit being so connected to the negative bias circuit that when the governor points are open the bias on said grids remains such as to prevent the tubes from energizing the coil, but

when the governor contacts are closed, the coil will be energized; a torque bias circuit also energized from said A. C. circuit and including a rectifier, said torque bias circuit being connected with the said two bias circuits so that upon undue rise of current in the A. C. circuit supplying the motor, said rectifier tubes will be prevented from energizing the coil regardless of the open or closed condition of said governor points.

1'1. In apparatus of the class described, a motor energized from an A. C. circuit, an eddycurrent clutch having a driving member connected to the motor and having a driven member connected to the load, a D C. field coil in said clutch, a rectifying tube feeding from said circuit into said coil, said tube having a grid, a grid circuit connecting the A. C. circuit with the grid characaterized by having a low wattage, and centrifugally operated speed responsive means in the low-wattage grid circuit responsive centrifugally to the speed exclusively of the driven member of said clutch, to make and break said grid circuit so that excessive speeds cause said tube to stop its current output. 7

18. In apparatus of the class described, a motor energized from an A. C. circuit, an eddycurrent clutch having a driving member' connected to the motor and having a driven member connected to the load, a D. C. field coil in said clutch, a rectifying tube feeding into said coil, said tube having an anode, a cathode and a grid, a low-wattage grid circuit, an anode transformer fed from the A. C. circuit and having a secondary connected to the anode, a grid transformer having a primary connected to one part of said A. C. circuit and a secondary connected to said grid circuit and to said cathode, and centrli'ugally operated speed responsive means in the primary of said grid transformer responsive centrifugally to increased speed exclusively of the driven mem ber of said clutch to deenergize the grid transformer, excessive speeds causing said tube to stop its current output and vice versa.

19. In apparatus of the class described, a motor energized from an A. C. circuit, an eddy-current clutch between the motor and the load, a D. C. field coil in. said clutch, at least one rectifying tube feeding into said coil, said tube having an anode, a cathode and a grid, a low-wattage circuit for the grid, an anode transformer having a secondary connected-to the anode, a grid transformer having a primary connected to one part of said A. C. circuit and a secondary connected to said grid circuit and to said cathode, a current transformer having a primary connected to another part of said A. C. circuit and having a secondary connected to said grid transformer in potential opposition, and speed responsive means in the primary of said grid transformer responsive to the speed of the load side of said clutch to deenergize the grid transformer, excessive speeds causing said tube to stop its current output.

ANTHONY WINTHER. 

